I. Field of the Invention
The invention concerns a blow mold for the production of bellows made of highly elastic plastic.
II. Description of the Prior Art
For bellows, especially those used for the construction of automobiles where, for instance, at the steering mechanism the bellows come in contact with oil, the demand exists for bellows made of a material, which is substantially resistant with regard to chemicals and also mechanical stress and strain, and which can be subjected to the constant stress and strain during use without danger of a premature destruction. Since rubber, which was used up to now, does not possess the necessary resistance, bellows have been used consisting of a plastic which possesses sufficient resistance and, in addition, is highly elastic. Such a plastic is, for instance, the so-called polyurethane. These bellows, which consist of plastic, are manufactured by means of blow molding employing customary methods, where the raw material represents a foil for bellows which are constructed in one level and where the raw material represents a tube for bellows which are constructed in the form of a hollow structure. The foil or the tube, respectively, is brought into contact with a mold by means of blow pressure. The forming surface of the mold corresponds to the surface of the bellows. The individual folds, which are connected to each other, for the known bellows have a triangular cross-section and linear side flanks. It is known that the blowing of the bellows results in a reduction of the wall thickness of the folds between the inner apex and the outer apex of each fold, because the foil or the tube respectively comes in contact with the linear side flanks of the fold as a result of the applied pressure. This results in a cooling of the freshly made bellows at the area of contact with the side flanks, and in this way a further stretching of the freshly made bellows is prevented. Therefore, the wall thickness at the upper apex of the fold is substantially smaller as compared to the wall thickness at the inner apex. Since the point of smallest wall thickness is the deciding factor for the durability and the resistance to stress and strain, a foil or a tube must be employed for blow molding with such an original wall thickness that the outer apex of each fold exhibits the desired wall thickness, despite the reduction of the wall thickness which occurs in the folds. This necessarily results in a relatively high consumption of raw material, which is actually unnecessary and which represents an economic disadvantage, as well as resulting in a prolonged production time. In addition, the removal of the blow-molded bellows from the mold is difficult because of its shape, and usually the bellows have to be removed manually from the mold, after the two half-molds have been opened. This leads to a further prolongation of the production time.
There exists a need to keep the raw material consumption for the production of such plastic bellows by means of blow molding as low as possible, without reducing the wall thickness of the outer apex of the fold to a thickness below the desired minimum thickness, and at the same time to make it possible to eject the bellows from the mold in order to automatize the production.